In recent years, research related to a light emitting device having an EL element as a self-luminous element has been actively carried out. Particularly, a light emitting device using an organic material as an EL material has been attracting attention. The light emitting device is also referred to as an EL display device. The EL element includes a layer containing an organic compound, which generates luminescence (electroluminescence) by being applied with an electric field (hereinafter, referred to as an EL layer); an anode; and a cathode. The luminescence generated in the layer containing the organic compound includes luminescence (fluorescence) that is generated upon returning of electrons to a ground state from excited singlet state and luminescence (phosphorescence) that is generated upon returning of electrons to a ground state from excited triplet state.
The EL element has a structure in which an EL layer is sandwiched between a pair of electrodes. The EL layer generally has a lamination structure. Typically, a lamination structure of “a hole transporting layer, a light emitting layer, and an electron transporting layer” is cited. The structure provides greatly high light-emitting efficiency and has been used in almost all light emitting devices that have been researched and developed now.
Alternatively, a structure composed by sequentially laminating a hole injecting layer, a hole transporting layer, a light emitting layer, and an electron transporting layer on an anode or a structure composed by sequentially laminating a hole injecting layer, a hole transporting layer, a light emitting layer, an electron transporting layer, and an electron injecting layer is also applicable. Fluorescent pigments and the like may be doped to the light emitting layer.
Either a low molecular weight material or a high molecular weight material can be used for forming these layers.
In addition, the EL layer is a generic term used to refer to all layers formed between a cathode and an anode. Therefore, all of each the above-mentioned hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, and electron injecting layer are included in the EL layer.
Further, a light emitting element composed of a cathode, an EL layer, and an anode is referred to as an EL element. There are two kinds for forming the EL element as follows: a system for forming an EL layer between two kinds of striped electrodes that run at right angles to one another (passive matrix system); and another system for forming an EL layer between a pixel electrode and a counter electrode arranged in matrix that are connected to a TFT (active matrix system). When the pixel density is increased, it has been considered that the active matrix system has an advantage over the simple matrix system since the active matrix can be driven at lower voltage for having switches in each pixel (or each dot).
Since the EL element is extremely and easily deteriorated by being oxidized or absorbing moisture due to existence of oxygen or moisture, there has been a problem that the light-emitting efficiency of the EL element is decreased or the lifetime thereof is shortened. Therefore, moisture and oxygen has been conventionally prevented from penetrating into the EL element as follows: the EL element is covered with an opposing substrate, dry air is filled thereinto, and a drying agent is further attached thereto. A substrate with the EL element formed thereon and the opposing substrate are adhered to each other with a sealing agent (for example, see Patent Document 1).
A step for adhering the substrate with the EL element formed thereon and the opposing substrate with the sealing agent is referred to as a sealing or a sealing step.
Further, since an EL material is damaged by UV irradiation, there has been a problem in which the light-emitting efficiency of a light emitting element is reduced and lifetime thereof is shortened.
The EL element has conventionally included a structure in which an electrode is formed as an anode over a substrate, an organic compound layer is formed on the anode, and a cathode is formed on the organic compound layer so that light generated in the organic compound layer is emitted toward a TFT through the anode, which is a transparent electrode (hereinafter, the structure is referred to as a bottom emission structure).
[Patent Document 1]: Japanese Patent Application Laid-Open No. 2002-352951
The EL element can be covered with the opposing substrate in the above-mentioned bottom emission structure. However, in the case of a structure in which an electrode is formed as an anode over a substrate, a layer containing an organic compound is formed on the anode, and a cathode that is a transparent electrode is formed on the layer containing the organic compound (hereinafter, the structure is referred to as a top emission structure), an opposing substrate made from a light shielding material cannot be used. Similarly, the same is true in the case of a dual emission structure in which light is simultaneously emitted upward and downward. As compared with the bottom emission structure, in the top emission structure and the dual emission structure, the number of material layers through which light emitted from the layer containing the organic compound passes can be reduced, thereby suppressing stray light generation between the material layers having different refractive indices. In the case of the bottom emission structure, it is necessary to pay minute attention to handling of a drying agent so as not to absorb moisture. Therefore, it has been necessary to encapsulate the drying agent quickly. Further, in the case of the top emission structure and the dual emission structure, when the drying agent is arranged on a pixel portion, the drying agent hinders display.
Further, the substrate with the EL element formed thereon and the opposing substrate are attached to each other with an UV-curable or a heat-curable sealing agent, wherein the EL element exists inside a space hermetically-sealed with the sealing agent, the opposing substrate, and the substrate. It is preferable that moisture and oxygen do not exist in the space and do not penetrate thereinto. When oxygen and moisture exist therein, a problem in which the EL element is deteriorated has been caused. The sealing agent is also referred to as a sealing material.
As compared with the heat-curable sealing agent, the UV-curable sealing agent is quickly cured using a device with smaller size, and therefore the UV-curable sealing agent has advantages in mass production. Therefore, there are many sealing devices for mass production each of which has only a function of UV irradiation as a function of curing the sealing agent. In the case of using such sealing devices, however, the heat-curable sealing agent cannot be used therein.
Further, since the EL element is damaged by UV irradiation and thermal shock, there has been a problem of decreasing the luminance for the EL element and a problem of shortening its lifetime.
When sealing materials before being cured are in contact with each other for a long time, the sealing materials are likely to be mixed with each other, which result in deformation. Further, since a mixed portion of the mixed sealing materials is not cured uniformly, the adhesive strength is likely to be reduced.
In order to overcome the foregoing problems, it is an object of the present invention as disclosed in the specification is to provide a light emitting device having a structure for preventing oxygen and moisture from penetrating into an EL element, and a method of manufacturing the same. Furthermore, with respect to the top-emission structure and the dual-emission structure in addition to the bottom-emission structure, it is an object of the present invention to encapsulate an EL element by uniformly curing all of the sealing materials without inserting a drying agent and damaging the EL element due to the UV irradiation even when a sealing device, which only has a function of UV irradiation, is used.
According to one aspect of the present invention, there is provided a light emitting device including a pixel portion having a plurality of EL elements between a pair of substrates, at least one of which has a light-transmitting property, wherein each of the plurality of EL elements includes: a first electrode; an orgamc compound layer on and in direct contact with the first electrode; and a second electrode on and in direct contact with the organic compound layer. The light emitting device further includes: a first sealing agent surrounding the pixel portion; and a second sealing agent formed in a region surrounded by the first sealing agent so as to cover the entire surface of the pixel portion, wherein the pair of substrates is attached to each other with the first sealing agent and second sealing agent. As for the first sealing agent, a sealing agent containing a gap material (such as a filer and a fine particle) for maintaining a gap between the pair of substrates can be used. As for the second sealing agent, a transparent sealing agent can be used. Light emitted from the EL element passes through the second sealing agent and one of the pair of substrates. A transparent sealing substrate is used as the one of the pair of substrates, and a substrate with the EL element formed thereon is used as another one of the pair of substrates so that the top emission structure can be completed by pasting these substrates to each other. In addition, light emitted from the EL element can transmit through another one of the pair of substrates with the EL element formed thereon as well as the second sealing agent and the one of the pair of substrates.
Since the height of the second sealing agent immediately after application is larger than that of the first sealing agent, when the two pieces of substrates are adhered to each other, the second sealing agent is pressed and spread so as to cover the pixel portion prior to the first sealing agent. At this moment, the pixel portion can be surely covered with the second sealing agent due to the arrangement of the pair of first patterns. Further, after the second sealing agent is spread on the entire surface of the pixel portion, the first sealing agent is subsequently spread. At this moment, gaps in the vicinity of each midpoint of the two sides for the second pattern formed along the pair of first patterns are filled, respectively. The second sealing agent is completely shielded from the outside air by the first sealing agent. Therefore, it is possible to prevent moisture and oxygen from reaching into the EL element by both of the first sealing agent and the second sealing agent.
After adhering the two pieces of substrates, the first sealing agent is cured first by being irradiated with UV light, and then the second sealing agent is subsequently cured by heating. Although the second sealing agent is heated for a long time to be cured, the second sealing agent is not mixed with the first sealing agent since the first sealing agent has been previously cured.
During the UV irradiation, the pixel portion is protected with a light-shielding mask and the like such that the pixel portion is not irradiated with UV light selectively.
When the two pieces of substrates are adhered to each other in the sealing step, a surface of the substrate is continuously and perpendicularly pressed in a direction of pressing the sealing materials between the substrates until the sealing materials are completely cured.
According to the invention as disclosed in the specification, it is not necessary to press the substrate for a long time in a heating step for curing the second sealing agent after the first sealing agent is cured. That is, the substrate has been necessary to be pressed conventionally until the sealing materials are cured in adhering the substrates in the sealing step. However, in the present invention, it is not necessary to press the substrates since the gap between the substrates is constantly maintained after the first sealing agent has been previously cured due to UV irradiation. Accordingly, the sealing step according to the present invention can be carried out in either a sealing device only for a UV-curable sealing agent or a sealing device for both the UV-curable sealing agent and a heat-curable sealing agent.
In the case of sealing an EL element for the top emission structure or the dual emission structure, the second sealing agent is cured by heating rather than by UV irradiation. Accordingly, the pixel portion is not damaged by the UV irradiation, and hence, the problems of decreasing luminance of the EL element and shortening lifetime thereof can be solved.
According to the invention as disclosed in the specification, in the sealing step of the top emission structure and the dual emission structure, the sealing agent covering the pixel portion can be cured without damaging the EL element due to UV irradiation even in the case of using a sealing device that only has a function of UV irradiation. As a result, a light emitting device with high reliability can be obtained.
Since the first sealing agent in the periphery of the pixel portion is cured at short times by being irradiated with UV light prior to the second sealing agent, after curing the first sealing agent, the first sealing agent and the second sealing agent are not mixed with each other even if they are in contact with each other for a long time. Therefore, the sealing materials are not deformed and the adhesive strength thereof is not degraded, thereby providing a light emitting device with higher reliability.
Furthermore, since the second pattern of the first sealing agent is continuously formed, the second pattern of the first sealing agent allows the second sealing agent to be shielded from the outside air completely. Consequently, it is possible to prevent moisture or oxygen from reaching into the EL element by both the first sealing agents and second sealing agent. As a result, a light emitting device with higher reliability can be obtained.